1. Field of the Inventions
The present inventions relate generally to orthopedic joint replacements, and more specifically, to a glenoid component.
2. Description of the Related Art
Anatomically, a joint is a more or less movable junction in the body of a subject of two or more bones. As used herein, the term is meant to include the different kinds of ligaments, tendons, cartilages, bursae, synovial membranes and bones comprising the mobile skeletal system of a subject in various quantities and configurations.
The shoulder joint is the body's most mobile joint, in that it can turn in many directions. The shoulder is a ball-and-socket joint made up of three bones: the upper arm bone (humerus), shoulder blade (scapula) and collarbone (clavicle). Two joints facilitate shoulder movement. The acromioclavicular (AC) joint joins one end of the collarbone with the shoulder blade; it is located between the acromion (the part of the scapula that forms the highest point of the shoulder) and the clavicle. The other end of the collarbone is joined with the breastbone (sternum) by the sternoclavicular joint. The glenohumeral joint, commonly called the shoulder joint, is a ball-and-socket type joint that helps move the shoulder forward and backward and allows the arm to rotate in a circular fashion or hinge out and up away from the body. The ball of the glenohumeral joint is the top, rounded portion of the humerus; the socket, or glenoid, is a dish-shaped part of the outer edge of the scapula into which the ball fits. The socket of the glenoid is surrounded by a soft-tissue ring of fibrocartilage (the glenoid labrum) that runs around the cavity of the scapula (glenoid cavity) in which the head of the humerus fits. The labrum deepens the glenoid cavity and effectively increases the surface of the shoulder joint, which helps stabilize the joint.
The bones of the shoulder are held in place by muscles, tendons (tough cords of tissue that attach the shoulder muscles to bone and assist the muscles in moving the shoulder) and ligaments (bands of fibrous tissue that connects bone to bone or cartilage to bone, supporting or strengthening a joint). A smooth, durable surface (the articular cartilage) on the head of the arm bone, and a thin lining (synovium) allows smooth motion of the shoulder joint. The joint capsule, a thin sheet of fibers that encircles the shoulder joint, allows a wide range of motion yet provides stability of the joint. The capsule is lined by a thin, smooth synovial membrane. The front of the joint capsule is anchored by three glenohumeral ligaments.
The rotator cuff, a structure composed of tendons and associated muscles that holds the ball at the top of the humerus in the glenoid socket, covers the shoulder joint and joint capsule. The rotator cuff provides mobility and strength to the shoulder joint. A sac-like membrane (bursa) between the rotator cuff and the shoulder blade cushions and helps lubricate the motion between these two structures.
The shoulder is an unstable joint easily subject to injury because of its range of motion, and because the ball of the humerus is larger than the glenoid that holds it. To remain stable, the shoulder must be anchored by its muscles, tendons and ligaments. Some shoulder problems arise from the disruption of these soft tissues due to injury or overuse, or underuse of the shoulder. Other problems can arise from degenerative processes.
For example, instability of the shoulder joint refers to situations that occur when one of the shoulder joints moves or is forced out of its normal position. The two basic forms of shoulder instability are subluxations and dislocations. A partial or incomplete dislocation of the shoulder joint (subluxation) means the head of the humerus is partially out of the socket (glenoid). A complete dislocation of the shoulder joint means that the head of the humerus is completely out of the socket. Anterior instability, for example, refers to a type of shoulder dislocation where the shoulder slips forward, meaning that the humerus moved forward and down out of its joint. Anterior instability may occur when the arm is placed in a throwing position Both partial and complete dislocation cause pain and unsteadiness in the shoulder joint. Patients with repeat dislocation usually require surgery.
Bursitis or tendonitis can occur with overuse from repetitive activities, which cause rubbing or squeezing (impingement) of the rotator cuff under the acromion and in the acromioclavicular joint. Partial thickness rotator cuff tears, most often the result of heavy lifting or fails, can be associated with chronic inflammation and the development of spurs on the underside of the acromion or the AC joint. Full thickness rotator cuff tears most often are the result of impingement.
Osteoarthritis and rheumatoid arthritis can cause destruction of the shoulder joint and surrounding tissue and degeneration and tearing of the capsule or rotator cuff. In osteoarthritis, the articular surface of the joint wears thin. Rheumatoid arthritis is associated with chronic inflammation of the synovium lining, which can produce substances that eventually destroy the inner lining of the joint, including the articular surface.
Shoulder replacement is recommended for subjects with painful shoulders and limited motion. The treatment options are either replacement of the head of the humerus or replacement of the entire socket. However, available treatment options are less than adequate in restoring shoulder joint function.
Just as muscles get stronger through use, the density and strength of bone varies with respect to the bone's load history. To ensure proper bone loading and good bone health, accurate implant placement, good bone fit, and restoration of a healthy anatomic position is critical.
The two major factors that contribute to articulation stability are soft tissue tension and radius of curvature of the glenoid. While some constraint is necessary for a stable joint, too much will increase the forces that contribute to glenoid loosing, one of the most significant problems in shoulder replacement. Because different activities require different levels of constraint in different areas of the glenoid, a simple spherical or dual radius surface may provide too much constraint in certain areas. When the joint is over-constrained, the excess forces that resist humeral head translation also increase glenoid loosening forces.
Optimum glenoid constraint may not be able to be achieved with a simple spherical surface. This principal is emphasized by the natural glenoid/labrum combination, which is not spherical and does not provide the same maximum constraint in all translation directions. Referring to FIG. 1, a currently available shoulder prosthesis glenoid component 10 has an articulating surface 12, which is essentially defined by a spherical or dual radius, fully concave geometry. As such, prior art glenoid components do not take into account the differing levels of constraint required for different activities or the varying curvature of the natural glenoid.
Moreover, since currently available glenoid components have fully concave articulating surfaces, as the humeral head translates, the contact point between the head and glenoid will approach the edge of the glenoid. At a certain point, as illustrated in FIG. 2, a load vector 20 being applied to the glenoid component 10 by a humeral head 22 will no longer pass through the glenoid bone 24, but will load the glenoid component 10 in an overhanging manner, significantly increasing loosening tendencies of the glenoid component 10.
Fixation of the glenoid component of a shoulder prosthesis is particularly important to the outcome of total shoulder reconstruction. Bone cement is commonly used to affix the glenoid component to the scapular neck, and pegs or keels are considered essential for fixation of cemented glenoid components. As shown in FIG. 3, a prior art glenoid component 10 is shown being attached to the glenoid bone 24. As illustrated, the glenoid component can have at least one peg 30. The geometry of the peg 30 typically consists of a substantially cylindrical peg body with various recesses 32 or protrusions for cement fixation. The glenoid bone 24 can have holes 36 that corresponded to the pegs. While this type of geometry may provide adequate fixation of the peg 30 within cement 40, it indiscriminately transfers loads to the cement mantle 40 and cement bone interface 42, creating very high stresses at the proximal edge 44 of the cement mantle. Since cement 40 and the cement bone interface 42 are weak in tension, this often results in the cement mantle 40 breaking free from the bone or breaking apart due to local high stresses.